1. Field of the Invention
This invention relates generally to direct access storage devices and, more particularly, to control of arm movement in disk drive devices.
2. Description of the Related Art
In a conventional computer data storage system having a rotating storage medium, such as a magnetic or magneto-optical disk, data is stored in a series of concentric or spiral tracks across the surface of the disk. A magnetic disk, for example, can comprise a disk substrate having a surface on which a magnetic material is deposited. The digital data stored on a disk is represented as a series of variations in magnetic orientation of the disk magnetic material. The variations in magnetic orientation, generally comprising reversals of magnetic flux, represent binary digits of ones and zeroes that in turn represent data. The binary digits must be read from and recorded onto the disk surface. A read/write head produces and detects variations in magnetic orientation of the magnetic material as the disk rotates relative to the head.
Conventionally, the read/write head is mounted on a disk arm that is moved across the disk by a servo. A disk drive servo control system controls movement of the disk arm across the surface of the disk to move the read/write head from data track to data track and, once over a selected track, to maintain the head in a path centered over the selected track. Maintaining the head centered over a track facilitates accurate reading and recording of data. Positioning read/write heads is one of the most critical aspects of recording and retrieving data in disk storage systems. With the very high track density of current disk drives, even the smallest head positioning error can potentially cause a loss of data that a disk drive customer wants to record or read. Accordingly, a great deal of effort is devoted to servo control systems.
Servo Control Systems
A servo control system generally maintains a read/write head in a position centered over a track by reading servo information recorded onto the disk surface. The servo information comprises a position-encoded servo pattern of high frequency magnetic flux transitions, generally flux reversals, that are pre-recorded in disk servo tracks. The flux transitions are recorded as periodic servo pattern bursts formed as parallel stripes in the servo tracks. When the read/write head passes over the servo pattern flux transitions, the head generates an analog signal whose repeating cyclic variations can be demodulated and decoded to indicate the position of the head over the disk. The position indicating information can be used to produce a corrective signal that is referred to as a position error sensing (PES) signal. The PES signal indicates which direction the head should be moved to remain centered over a selected track and properly read and write data.
There are a variety of methods for providing servo track information to a disk servo control system. In the dedicated servo method, one surface of a disk is completely recorded with servo track information. Typically, a servo head is positioned over the dedicated servo disk surface in a fixed relationship relative to multiple data read/write heads positioned over one or more other data disk surfaces. The position of the servo head relative to the dedicated disk surface is used to indicate the position of the multiple data read/write heads relative to their respective disk surfaces.
Another method of providing servo track information is known as the sector servo method. In the sector servo method, each disk surface includes servo track information and customer data recorded in concentric or spiral tracks. The tracks on a sector servo disk surface are partitioned by sectors having a short servo track information area followed by a data area. The servo track information area typically includes a sector marker, track identification data, and a servo burst pattern. The sector marker indicates to the data read/write head that servo information immediately follows in the track. The servo read head is typically the same head used for reading data.
FIG. 1 is a schematic representation of a conventional disk drive storage system 100 that includes one or more individual disks 102 on which are deposited a magnetic recording material for storing magnetically encoded information. The disk drive 100 also includes an actuator 104 with a read/write head 106. An actuator motor 108 pivots the actuator 104, thereby changing the position of the read/write head 106 with respect to concentric tracks 110 of data contained on the disk 102. The operation of the disk drive 100 is managed by a disk drive controller 112, which also serves as an interface between the disk drive 100 and a host computer 113.
The controller 112 includes a readback signal pre-amplifier 116 ("pre-amp"), which receives electrical representations of the flux changes sensed by the read/write head 106 from the disk 102. The pre-amp 116 serves a dual purpose by amplifying either data signals or servo signals, depending on whether the read/write head 106 is positioned over stored customer data or over servo pattern data, respectively. Thus, the amplified signal from the pre-amp 116 is directed to two processing channels: a servo channel 118 and a customer data channel 120. A write circuit 117 is provided to supply the read/write head 106 with customer data signals from the data channel.
The data channel 120 generally reads and writes data to and from the disk 102 in response to requests from the host computer 113 to read or write the data. The write circuit 117 is connected only to the customer data channel. The pre-amp 116, when operating in conjunction with the customer data channel 120, amplifies the disk readback signal from the read/write head 106 and directs the readback signal to an automatic gain control and filter circuit 121. A data pulse detector 122 receives the analog readback signal from the circuit 121 and forms digital data pulses corresponding to the analog signals. Next, a pre-host processor 124 converts the data pulses into formatted data strings that are compatible with the host computer 113. The components of the data channel 120 also operate in reverse order to write customer data to the disk 102.
The servo channel 118 generally reads servo data from the disk 102 to aid in properly positioning the read/write head 106. When operating in conjunction with the servo channel 118, the pre-amp 116 amplifies servo signals produced when the read/write head 106 senses servo patterns. The servo channel 118 includes an automatic gain control (AGC) and filter circuit 126, which may comprise any one of various known circuits for automatically adjusting the readback signal gain and filtering it. Next, a demodulator/decoder 128 receives the filtered readback signal and processes the information to derive a position error sensing (PES) signal, which is related to the position of the read/write head 106 with respect to the desired track center and is indicative of the read/write head position error. The PES signal is then used by a servo controller 130 to generate an input signal that, when provided to the actuator 104, controls the position of the read/write head 106.
The servo pattern is recorded into, and read from, tracks across the disk 118. In FIG. 1, circular, parallel lines 164 designate servo tracks of the disk, which are divided into sectors that are represented by radial lines 166. The servo tracks can include several repeated cycles of a servo pattern and can encompass one or more tracks of customer data.
Servo Signals
FIG. 2 shows a representation of various servo pattern bursts 138, 139, 140, 141 recorded on the surface of the disk 102. FIG. 2 also illustrates an amplitude-type servo readback signal 144 that is generated when the read/write head 106 is positioned above a first track 136. Each servo burst 138-141 is sensed and processed to provide servo signals that guide the read/write head 106 along one of the tracks 136-137. Those skilled in the art will recognize that the FIG. 2 servo bursts 138, 139, 140, 141 form a quadrature pattern, the bursts being commonly referred to by the designations A, B, C, and D, respectively. Although the A and B bursts 138, 139 most directly serve to guide the read/write head 106 in following the track 136, these two bursts also provide position information that is useful in guiding the head 106 along more remote tracks, such as the track 137. The illustrated servo pattern is referred to as an amplitude-type pattern because the amplitude of the readback signal is greatest when the head 106 is centered over one of the servo bursts, and decreases in amplitude as the head is moved away from the longitudinal center of a burst. Thus, because the head 106 is shown in FIG. 2 tracking a path centered over the C burst along the track 136, the portion of the readback signal 144 with the greatest amplitude is generated when the head is over the servo burst C.
Those skilled in the art will appreciate that the decoded servo signal 144 can be adversely effected by a variety of factors, including readback signal noise, run out error of the disk servo pattern, dynamics of the arm 104, and physical vibrations due to shock or other jostling of the disk drive system 100. Any one of these factors may cause an inaccurate servo signal, which can lead to mistracking, but the potentially most damaging source of error, and most difficult to overcome, is signal error due to shock.
Responding to Shock
As noted above, a disk drive system is sometimes subjected to a physical shock that can cause mistracking of the read/write head 106. When that occurs, read or write errors can occur. A misread is not especially problematic, because error checking circuitry generally results in an immediate re-read operation so that data is correctly read. A shock-generated write error, however, can result in non-recoverable data errors. To prevent such occurrences, some disk drive systems include a hardware shock sensor to detect when a shock occurs and generate a temporary write inhibit command that prevents writing to the disk when a shock has been detected.
Often, disk drives rely only on the PES signal from the surface of interest to detect a shock event. As track pitch increases, this provides increasingly inadequate coverage, since smaller off-track motion now has more serious consequences. The principle shortcoming is that time between PES samples is too long for the required protection, and decreasing that time requires writing more PES information and less customer data. Thus such a protection scheme negatively impacts the data capacity of the drive.
A hardware shock sensor typically includes one or more accelerometers that sense when forces are applied to the disk drive system 100 components, such as a housing of the system. Other processing equipment detects the signal generated by the accelerometers, processes the signals, and determines if a write inhibit command should be issued. Such detecting devices and processing equipment can add greatly to the cost of producing the disk drive system. Moreover, such hardware-implemented shock detectors may not always provide a precise indication of when a shock has occurred. For example, the accelerometers detect acceleration forces being experienced on whatever disk system component to which they are attached, but shock at that component may not indicate shock at the read/write head, which is where mistracking would occur. Attaching an accelerometer to the disk arm would provide greater accuracy, but increases the total arm mass that must be controlled by the servo.
From the discussion above, it should be apparent that there is a need for a direct access storage device that efficiently and accurately detects physical shocks. The present invention fulfills this need.